Attempts to use liquid crystal elements which utilize electrooptic effect of a twisted nematic (TN) liquid crystal phase (referred to as "TN liquid crystal elements" hereinafter) as display elements of display devices for transient images of moving objects (moving images) have been heretofore made.
The TN liquid crystal elements includes a liquid crystal cell wherein a liquid crystal material capable of exhibiting a twisted nematic (TN) liquid crystal phase (said cell being referred to as "TN liquid crystal cell" hereinafter) is filled in a cell gap.
Most of the recent TN liquid crystal cells used in full color display devices for moving images are driven on an active matrix driving system utilizing TFT (thin film transistor) or MIM (metal insulator metal).
The TN liquid crystal elements having such liquid crystal cells as mentioned above can display images of excellent gradation property, so that they are favorably used for displaying full color images. The term "gradation property" means such a property that brightness intensities between the maximum brightness (white) and the minimum brightness (black) can be stepwise discriminated. For example, by the term "16-gradation" is meant that the brightness of 16 steps can be discriminated from each other between the maximum brightness and the minimum brightness. As the number of the discriminating brightness steps increases, the gradation property becomes better.
The TN liquid crystal elements, however, have a long (slow) electrooptic response time of several tens seconds. Therefore, the display devices using the TN liquid crystal elements cannot follow images of quick motions. Further, the range of the angle at which an image displayed using the TN liquid crystal element is visible is narrow, and when the images displayed by the TN liquid crystal element is observed with an angle outside of a particular range of angle, a problem of the image with reversed gradation or a problem of change in hue of the image takes place.
When the display elements as mentioned above are driven on the active matrix system, the frame frequency in the image display can be set at, for example, not less than 60 Hz, but the electrooptic switching time generally becomes at least several tens msec and occasionally becomes about 200 msec. Therefore, it is difficult to smoothly display moving images by means of the display devices for moving images using the TN liquid crystal elements.
In contrast therewith, liquid crystal elements utilizing electrooptic effect of a ferroelectric liquid crystal phase (referred to as "ferroelectric liquid crystal elements" hereinafter) and liquid crystal elements utilizing electrooptic effect of an antiferroelectric liquid crystal phase (referred to as "antiferroelectric liquid crystal elements" hereinafter) have extremely shorter electrooptic response time than that of the TN liquid crystal elements, and these elements have an advantage in that the range of an angle at which an Image displayed by the elements is visible is wide.
The ferroelectric liquid crystal phase reveals layer structures shown in, for example, FIG. 47(a) to FIG. 47(c).
FIG. 47(a) schematically illustrates an orientation state of liquid crystal molecules 101 which form an antiferroelectric liquid crystal phase between electrodes 102 and 102' provided on respective substrates 106 and 106' of a liquid crystal cell. In general, the electrode 102 and the electrode 102' are formed on one surface of the substrate 106 and one surface of the substrate 106', respectively. When the liquid crystal molecules 101 are orientated, the major axes of the liquid crystal molecules 101 are substantially parallel to the electrodes 102, 102', and the liquid crystal molecule major axes which are parallel to each other gather to form liquid crystal layers 103 perpendicular to the electrodes 102, 102'. When an orientation film (not shown) to control orientation directions of the liquid crystal molecules 101 is provided on the surface of one or both of the electrodes 102, 102', the liquid crystal layers 103 are in contact with the orientation film(s) with keeping a state as shown in FIG. 47(b) or FIG. 47(c). That is, the major axes of the liquid crystal molecules 101 which form the liquid crystal layers 103 have fixed tilt angles to a line which is perpendicular to the boundaries between the adjacent liquid crystal layers 103. The direction of spontaneous polarization of each liquid crystal molecule 101 is perpendicular to the surfaces of the electrodes 102, 102', however, the direction of the spontaneous polarization 104 shown in FIG. 47(b) and the direction of the spontaneous polarization 105 shown in FIG. 47(c) are opposite to each other. The orientation states of the liquid crystal molecules shown in FIG. 47(b) and FIG. 47(c) are both stable.
The ferroelectric liquid crystal element has two kinds of electrooptically stable states corresponding to the orientation states of the liquid crystal molecules shown in FIG. 47(b) and FIG. 47(c).
This has been reported by Clerk, et al., and they have proposed use of the ferroelectric liquid crystal elements as display devices.
As for the ferroelectric liquid crystal element, when a pulse voltage having a pulse width of several tens .mu.sec is applied between the electrodes 102, 102' of the liquid crystal cell, one of the bistable states shown in FIG. 47(b) and FIG. 47(c) is selected to modulate the light. The state thus selected is maintained even after application of voltage is stopped. Accordingly, the ferroelectric liquid crystal element has a capacity for memorizing use.
When a direct voltage is applied between the electrodes 102, 102' of the liquid crystal cell, the ferroelectric liquid crystal element does not show a clear threshold value in the voltage-light quantity correlation, but when a pulse voltage is applied, the ferroelectric liquid crystal element not only shows a clear threshold value in the voltage-light quantity correlation but also has a capacity for memorizing use. For this reason, driving of the liquid crystal cell of the ferroelectric liquid crystal element on a simple matrix system has been studied.
Differently from the ferroelectric liquid crystal element, the antiferroelectric liquid crystal element found by Chandany, et al. (Jpn. J. Appl. Phys., 28, L1261, 1989) includes a liquid crystal cell wherein a liquid crystal material capable of exhibiting an antiferroelectric liquid crystal phase (referred to as "antiferroelectric liquid crystal material" hereinafter) is filled in the cell gap.
As for the antiferroelectric liquid crystal element, the antiferroelectric liquid crystal material filled in the cell gap of the liquid crystal cell is in an antiferroelectric liquid crystal phase when the antiferroelectric liquid crystal element is electrooptically changed.
The antiferroelectric liquid crystal phase reveals layer structures shown in FIG. 48(a) to FIG. 48(d).
The orientation state of the liquid crystal molecules 201 shown in FIG. 48(a) is the same as that of the liquid crystal molecules 101 shown in FIG. 47(a); the orientation state of the liquid crystal molecules 201 shown in FIG. 48(b) is the same as that of the liquid crystal molecules 101 shown in FIG. 47(b); and the orientation state of the liquid crystal molecules 201 shown in FIG. 48(d) is the same as that of the liquid crystal molecules 101 shown in FIG. 47(c).
In the antiferroelectric liquid crystal state shown in FIG. 48(c), the directions of spontaneous polarization 104 of the liquid crystal molecules in one liquid crystal layer 103 and the directions of spontaneous polarization 105 of the liquid crystal molecules in the adjacent liquid crystal layer 103 are opposite to each other, and the intensity of the spontaneous polarization as a whole becomes zero.
If the electrodes 102, 102' of the liquid crystal cell are transparent electrodes and if a low-frequency triangular wave voltage of about 0.1 Hz is applied between the electrodes 102, 102', the quantity of the transmitted light released (output) from the antiferroelectric liquid crystal element forms a double hysteresis curve shown in FIG. 49. Besides, if a direct voltage is applied between the electrodes 102, 102', the antiferroelectric liquid crystal element shows a clear threshold value in the voltage-light quantity correlation. For these reasons, driving of the liquid crystal cell of the antiferroelectric liquid crystal element on a simple matrix system has been studied.
Since the above-mentioned ferroelectric liquid crystal element and antiferroelectric liquid crystal element have quick electrooptic response time, it is feasible to drive the liquid crystal cells of these elements on a simple matrix system to display moving images. Moreover, the range of the angle at which the image thus displayed is visible is extremely wider than that of an image displayed by the TN-liquid crystal element, so that the image displayed using the ferroelectric or antiferroelectric liquid crystal element hardly suffers lowering of contrast even if the image is observed obliquely, said lowering of contrast being a serious problem in the TN liquid crystal element.
The liquid crystal element utilizing electrooptic effect of the antiferroelectric liquid crystal phase (referred to as "antiferroelectric liquid crystal element" hereinafter) can electrooptically control brightness of the transmitted light and has an electrooptic switching time of several tens to several hundreds .mu.sec, as described by Jono, et al. in "Jpn. J. Appl. Phys." Vol. 29, L107 (1990), and therefore electrooptic switching can be made at a higher speed than the TN liquid crystal element.
At the International Ferroelectric Liquid Crystal Conference (Tokyo) in 1993, Nippon Denso Co., Ltd. and Citizen Co., Ltd. exhibited trial manufactures of antiferroelectric liquid crystal elements which were able to be driven on a simple matrix system using the above-mentioned properties. The trial manufactures have proved that use of the hysteresis of the antiferroelectric liquid crystal makes it possible to drive a display element of multi-scanning line display system on a simple matrix system.
The liquid crystal cell of the antiferroelectric liquid crystal element can be driven on a simple matrix system to display an image having gradation as described above, but in the existing circumstances, it is ditficult to obtain the gradation enough to form a good full color image.
That is, because the electrooptic response speed of the display device using the antiferroelectric liquid crystal element is insufficient, the frame frequency in the image display cannot be increased to 60 Hz or more, and therefore moving images of natural motion cannot be displayed. In case that the conventional antiferroelectric liquid crystal element is driven on an active matrix system, if a voltage is applied between the electrodes of the liquid crystal cell, the quantity of the light output from the liquid crystal element having the liquid crystal cell (e.g., quantity of the polarized light in the given polarization direction) steeply varies at the voltage of a particular intensity (threshold voltage). Therefore, it has been hitherto considered that in the antiferroelectric liquid crystal element the mere electrooptic discrimination of the output light between the brightness and the darkness is basically possible and that the antiferroelectric liquid crystal element outputs only a light having two kinds of light quantities corresponding to the brightness and the darkness.
Japanese Patent Laid-Open Publication No. 194626/1994 proposes an antiferroelectric liquid crystal element having such a property that, in case that the liquid crystal cell of the element is driven on an active matrix system, if a voltage is applied between the electrodes 102, 102' of the liquid crystal cell, the light quantity varies correspondingly to the applied voltage to form a double hysteresis curve with small hysteresis width as shown in FIG. 50.
In such circumstances as mentioned above, the present inventors have further studied the smectic liquid crystal compositions, and as a result, have found that, if a liquid crystal element is made using a liquid crystal cell wherein a specific smectic liquid crystal composition is filled between electrodes and if a voltage is applied between the electrodes of the liquid crystal cell, the quantity of the polarized light output from the liquid crystal element in the given polarization direction varies continuously over a wide voltage range. The present inventors have also found that when the liquid crystal element having the above properties is used as a display element, an image of excellent gradation can be obtained by merely controlling the intensity of the voltage applied between the electrodes of the liquid crystal cell, and therefore the liquid crystal element is advantageously used for displaying moving images. Further, the present inventors have found that, in order to display an image of high gradation by driving the liquid crystal cell of the antiferroelectric liquid crystal element on an active matrix system, the small hysteresis is not necessarily sufficient, and additionally a specific parameter G defined below should be made as small as possible. The specific parameter G is defined by a maximum light quantity T.sub.max and a minimum light quantity T.sub.min of a light output from the antiferroelectric liquid crystal element, a light quantity T.sub.0 of a light output from the antiferroelectric liquid crystal element when no voltage is applied between the electrodes of the liquid crystal cell, an area S enclosed with the double hysteresis curve formed when a positive or negative voltage is applied between the electrodes of the liquid crystal cell, and a minimum voltage .vertline.V.vertline..sub.min at which the light quantity T.sub.max is obtained.
It is an object of the present invention to provide a smectic liquid crystal composition capable of imparting voltage gradation property to a liquid crystal element.
It is another object of the invention to provide a liquid crystal composition capable of imparting excellent voltage gradation property to a liquid crystal element.
It is a further object of the invention to provide a liquid crystal composition and a smectic liquid crystal composition by the use of which images of excellent gradation can be displayed when elements using the compositions are driven on an active matrix system.
It is a still further object of the invention to provide a liquid crystal element comprising any of the above liquid crystal compositions and a method of driving the element.